Preterm birth is one of the main causes of neonatal and infant morbidity and mortality worldwide. In Uruguay, preterm birth accounts for 11.8% of all births and it is associated with the majority of neonatal deaths and severe neonatal morbidity, as well as with long-term morbidity. Some studies in Latin America have shown that preterm birth rates have notoriously increased during the last years as a consequence of the increase of medically indicated preterm births as well as population risk factors changes. The main objective of this study is to determine the risk factors for preterm birth in the low social economic status (SES) population from Uruguay and to examine the risk factors for medically indicated preterm births compared to the risk factors for spontaneous preterm births and for preterm births with premature rupture of membranes (PPROM) in the same population. We hypothesize that the risk factors for medically indicated preterm birth are different from the risk factors for a spontaneous preterm birth and PPROM. The existence of such differences is to be considered supportive of the hypothesis that medically indicated preterm birth is a different entity that should be separate from the other two subtypes of preterm births when researchers are approaching the topic of prematurity. The proposed investigation is a population-based case-control study in which cases will be defined by medically indicated preterm births, spontaneous preterm births and PPROM, and the controls will involve term births. During a period of time of 40 months, approximately 1,280 preterm births (cases) and 2,560 term births (controls) will be recruited in this study. All women recruited will have a complete questionnaire designed to collect data about specific risk factors. Two controls will be selected per each case, and an innovation of this case-control study will be that one control will be randomly selected from the same day of the preterm birth's delivery (traditional control selection) and a second control will be randomly selected from the due date for the corresponding case (newly proposed control). The comparison of both types of controls will offer a great opportunity to study a methodological issue that has not been yet approached in the medical literature, regarding control selection in perinatal case-control studies. The primary aims of this study will help identify modifiable risk factors to design prevention programs and possible future strategies to potentially reduce preterm birth rates. The long-term goal of this research program is to develop intervention strategies that are based on the risk factors identified in the proposed study resulting in decreased morbidity and mortality.